1. Field of the Invention
This invention relates to a multicolor printing method wherein a color image is separated into a plurality of colors at every picture element unit, thereby enabling multicolor printing using printing methods such as thermal transfer, ink jet or the like.
2. Description of the Prior Art
Heretofore, a tone gradation reproduction method known as the dither method has been used. In the dither method, the density of a plurality of black picture elements in a minute unit area is changed so as to create the effect of multiple tone gradation in a binary type of black-and-white printing. More specifically, an original image which is to be reproduced by the dither method is divided into a plurality of unit square regions each consisting of n.times.n picture elements (an n.times.n dot matrix). Threshold levels for deciding between black or white at each picture element are made to differ from one another over the n.sup.2 picture elements in each unit square region. In this case, the threshold levels of the plural picture elements in each unit square region can be viewed as defining an n.times.n matrix (which is called a dither matrix or dither pattern). Such a dither method has widely been utilized also in multicolor printing wherein the image is separated into three primary colors and differing threshold values are used for deciding between color-on or color off.
Conventional multicolor printing systems using the dither method typically employ the same dither pattern for each color. Accordingly, when ink layers of different colors are superimposed one upon another at the same picture element, the resulting printed image produces color saturation and brightness effects different from those of the original image. Further, there is an additional disadvantage in superimposing colors using the same dither pattern in combination with thermal transfer printing. In that case, fixation becomes difficult and produces inferior reproduced picture quality.
The present invention overcomes the above disadvantages and provides a multicolor printing method by which reproduced picture quality can be improved.